1. Field of the Invention
The present invention relates to a light emitter for a display for use in electronic products and a method of forming the light emitter and display.
2. Prior Art
Modern consumer electronics require cheap, high-contrast displays with good power efficiency and low drive voltages. Particular applications include displays for mobile phones and hand-held computers.
Conventional displays comprise twisted nematic liquid crystal displays (TN-LCDs) with active matrix addressing and super-twisted nematic liquid crystal displays (STN-LCDs) with multiplex addressing. These however require intense back lighting which presents a heavy drain on power. The low intrinsic brightness of LCDs is believed to be due to high losses of light caused by the absorbing polarizers and filters which can result in external transmission efficiencies of as low as 4%.
The Applicants have now devised a new kind of light emitter for a display which offers the prospect of lower power consumption and/or higher brightness. The display utilises an alternative light source which can in embodiments be used instead of the conventional polarizers and/or back light. The alternative light source comprises a light emitting polymer or polymer network aligned on a photoalignment layer. The combination of this alternative lighting source with existing LCD technology offers the possibility of low-cost, bright, portable displays with the benefits of simple manufacturing and enhanced power efficiency.
According to one aspect of the present invention there is provided a light emitter for a display comprising a photoalignment layer; and aligned on said photoalignment layer, a light emitting polymer.
The photoalignment layer is comprised of materials that photoalign (e.g. by cross-linking) to form anisotropic layers when polarised light (e.g. UV) is applied.
The photoalignment layer typically comprises a chromophore attached to a sidechain polymer backbone by a flexible spacer entity. Suitable chromophores include cinnamates or coumarins, including derivatives of 6 or 7-hydroxycoumarins. Suitable flexible spacers comprise unsaturated organic chains, including e.g. aliphatic, amine or ether linkages.
An exemplary photoalignment layer comprises the 7-hydroxycoumarin compound having the formula: 
Other suitable materials for use in photoalignment layers are described in M. O""Neill and S. M. Kelly, J. Phys. D. Appl. Phys. [2000], 33, R67.
In aspects, the photoalignment layer is photocurable. This allows for flexibility in the angle at which the light emitting polymer (e.g. as a liquid crystal) is alignable and thus flexibility in its polarization characteristics.
The photalignment layer may also be doped with a hole transport compound, that is to say a compound which enables hole transport within the photoalignment layer such as a triarylamine. Examples of suitable triarylamines include those described in C. H. Chen, J. Shi, C. W. Tang, Macromol Symp. [1997] 125, 1.
An exemplary hole transport compound is 4,4xe2x80x2,4xe2x80x3-tris[N-(1-napthyl)-N-phenyl-amino]triphenylamine which has the formula: 
In aspects, the hole transport compound has a tetrahedral (pyramidal) shape which acts such as to controllably disrupt the alignment characteristics of the layer.
In one aspect, the photoalignment layer includes a copolymer incorporating both linear rod-like hole-transporting and photoactive side chains.
Suitably, the light emitting polymer is a polymer having a light emitting chromophore. Suitable chromophores include fluorene, vinylenephenylene, anthracene and perylene. Useful chromophores are described in A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem. Int. Ed. Eng. [1998], 37, 402.
Suitably, the light emitting polymer is a liquid crystal which can be aligned to emit polarised light. A suitable class of polymers is based on fluorene.
In one aspect, the light emitting polymer comprises an organic light emitting diode (OLED) such as described in S. M. Kelly, Flat Panel Displays: Advanced Organic Materials, RSC Materials Monograph, ed. J. A. Connor, [2000]; C. H. Chen, J. Shi, C. W. Tang, Macromol Symp. [1997] 125, 1; R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos, J. L. Bredas, M. Logdlund, W. R. Salaneck, Nature [1999] 397, 121; M. Grell, D. D. C. Bradley, Adv. Mater. [1999]11, 895; N. C. Greenman, R. H. Friend Solid State Phys. [1995] 49, 1.
OLEDs may be configured to provide polarized electroluminescence.
The reactive mesogen (monomer) typically has a molecular weight of from 400 to 2,000. Lower molecular weight monomers are preferred because their viscosity is also lower leading to enhanced spin coating characteristics and shorter annealing times which aids processing. The light emitting polymer typically has a molecular weight of above 4,000, typically 4,000 to 15,000.
The light emitting polymer typically comprises from 5 to 50, preferably from 10 to 30 monomeric units.
The light emitting polymer is aligned on the photoalignment layer. Suitably, the photoaligned polymer comprises uniaxially aligned chromophores. Typically light polarization ratios of 30 to 40 are required, but with the use of a clean up polarizer ratios of 10 or more can be adequate for display uses.
In aspects, the light emitting polymer is formed by a polymerization process. Suitable processes involve the polymerization of reactive mesogens (e.g. in liquid crystal form) via photo-polymerization or thermal polymerization of suitable end-groups of the mesogens. In preferred aspects, the polymerization process results in cross-linking e.g. to form an insoluble, cross-linked network.
The polymerization process can in a preferred aspect be conducted in situ after deposition of the reactive mesogens on the photoalignment layer by any suitable deposition process including a spin-coating process.
In a preferred polymerization process, the light emitting polymer is formed by photopolymerization of reactive mesogens having photoactive end-groups.
Suitable reactive mesogens have the following general structure:
Bxe2x80x94Sxe2x80x94Axe2x80x94Sxe2x80x94Bxe2x80x83xe2x80x83(general formula 1)
wherein
A is a chromophore;
S is a spacer; and
B is an endgroup which is susceptible to radical photopolymerisation.
The polymerisation typically results in a light emitting polymer comprising arrangements of chromophores (e.g. uniaxially aligned) spaced by a crosslinked polymer backbone. The process is shown schematically in FIG. 1 from which it may be seen that the polymerisation of reactive monomer 10 results in the formation of crosslinked polymer network 20 comprising crosslink 22, polymer backbone 24 and spacer 26 elements.
Suitable chromophore (A) groups have been described previously.
Suitable spacer (S) groups comprise organic chains (e.g. unsaturated), including e.g. flexible aliphatic, amine or ether linkages. Aliphatic spacers are preferred. The presence of spacer groups aids the solubility and lowers the melting point of the light emitting polymer which assists the spin coating thereof.
Suitable endgroups are susceptible to photopolymerization (e.g. by a process using UV radiation, generally unpolarized). Preferably, the polymerization involves cyclopolymerization (i.e. the radical polymerization step results in formation of a cyclic entity).
A typical polymerization process involves exposure of a reactive mesogen of general formula 1 to UV radiation to form an initial radical having the general formula as shown below:
Bxe2x80x94Sxe2x80x94Axe2x80x94Sxe2x80x94Bxe2x80xa2xe2x80x83xe2x80x83(general formula 2)
wherein A, S and B are as defined previously and Bxe2x80xa2 is a radicalised endgroup which is capable of reacting with another B endgroup (particularly to form a cyclic entity). The Bxe2x80xa2 radicalised endgroup suitably comprises a bound radical such that the polymerisation process may be sterically controlled.
Suitable endgroups include dienes such as 1,4, 1,5 and 1,6 dienes. The diene functionalities may be separated by aliphatic linkages, but other inert linkages including ether and amine linkages may also be employed.
Methacrylate endgroups have been found to be less suitable than dienes because the high reactivity of the radicals formed after the photoinitiation step can result in a correspondingly high photodegradation rate. By contrast, it has been found that the photodegradation rate of light emitting polymers formed from dienes is much lower. The use of methacrylate endgroups also does not result in cyclopolymerization.
Where the endgroups are dienes the reaction typically involves cyclopolymerization by a sequential intramolecular and intermolecular propagation: A ring structure is formed first by reaction of the free radical with the second double bond of the diene group. A double ring is obtained by the cyclopolymerization which provides a particularly rigid backbone. The reaction is in general, sterically controlled.
Suitable reactive mesogens have the general formula: 
wherein R has the general formula:
Xxe2x80x94S2xe2x80x94Yxe2x80x94Z
and wherein
Xxe2x95x90O, CH2 or NH and preferably Xxe2x95x90O;
S2=linear or branched alkyl or alkenyl chain optionally including a heteroatom (e.g. O, S or NH) and preferably S2=a linear alkyl chain;
Yxe2x95x90O, CO2 or S and preferably Yxe2x95x90CO2; and
Z=a diene (end-group) and preferably Z=a 1,4, 1,5 or 1,6 diene.
Exemplary reactive mesogens have the general formula: 
An exemplary reactive mesogen has the formula: 
All of Compounds 3 to 6 exhibit a nematic phase with a clearing point (N-I) between 79 and 120xc2x0 C.
Other suitable exemplary reactive mesogens have the general formula: 
wherein n is from 2 to 10, preferably from 3 to 8 and wherein, as above, R has the general formula:
Xxe2x80x94S2xe2x80x94Yxe2x80x94Z
and wherein
Xxe2x95x90O, CH2 or NH and preferably Xxe2x95x90O;
S2=linear or branched alkyl or alkenyl chain optionally including a heteroatom (e.g. O, S or NH) and preferably S2=a linear alkyl chain;
Yxe2x95x90O, CO2 or S and preferably Yxe2x95x90CO2; and
Z=a diene (end-group) and preferably Z=a 1,4, 1,5 or 1,6 diene.
Suitably, R is as for any of Compounds 3 to 6, as shown above.
A particular class of exemplary reactive mesogens has the formula: 
wherein:
n is from 2 to 10, preferably from 3 to 8; and
m is from 4 to 12, preferably from 5 to 11.
Still further suitable exemplary reactive mesogens have the general formula: 
wherein Axe2x95x90H or F
and wherein, as above, R has the general formula:
Xxe2x80x94S2xe2x80x94Yxe2x80x94Z
and wherein
Xxe2x95x90O, CH2 or NH and preferably Xxe2x95x90O;
S2=linear or branched alkyl or alkenyl chain optionally including a heteroatom (e.g. O, S or NH) and preferably S2=a linear alkyl chain;
Yxe2x95x90O, CO2 or S and preferably Yxe2x95x90CO2; and
Z=a diene (end-group) and preferably Z=a 1,4, 1,5 or 1,6 diene.
Suitably, R is as for any of Compounds 3 to 6, as shown above.
Particular exemplary reactive mesogens of this type have the formula: 
In aspects, the preferred photopolymerization process can be conducted at room temperature, thereby minimizing any possible thermal degradation of the reactive mesogen or polymer entities. Photopolymerization is also preferable to thermal polymerization because it allows subsequent sub-pixellation of the formed polymer by lithographic means.
Further steps may be conducted prior to the polymerization process including doping of the reactive mesogen. The dopant may in aspects comprise a further reactive monomer capable of co-polymerization with the reactive mesogen.
Further steps also may be conducted subsequent to the polymerization process including doping and the addition of other layers (as described in more detail below).
The light emitting polymer may be aligned by a range of methods including mechanical stretching, rubbing, and Langmuir-Blodgett deposition. Mechanical alignment methods can however lead to structural degradation. The use of rubbed polyimide is a suitable method for aligning the light emitting polymer especially in the liquid crystal state. However, standard polyimide alignment layers are insulators, giving rise to low charge injection for OLEDs.
The susceptibility to damage of the alignment layer during the alignment process can be reduced by the use of a non-contact photoalignment method. In such methods, illumination with polarized light introduces a surface anisotropy to the alignment layer and hence a preferred in-plane orientation to the overlying light emitting polymer (e.g. in liquid crystal form).
The aligned light emitting polymer is in one aspect in the form of an insoluble nematic polymer network. Cross-linking has been found to improve the photoluminescence properties.
M. O""Neill, S. M. Kelly J. Appl. Phys. D [2000] 33, R67 provides a review of photalignment materials and methods.
The emitter herein may include additional layers such as carrier transport layers. The presence of an electron-transporting polymer layer (e.g. comprising an oxadiazole ring) has been found to increase electroluminescence.
An exemplary electron transporting polymer has the formula: 
Pixellation of the light emitter may be achieved by selective photopatterning to produce red, green and blue pixels as desired. The pixels are typically rectangular in shape. The pixels typically have a size of from 1 to 50xc2x0 xcexcm. For microdisplays the pixel size is likely to be from 1 to 50 xcexcm, preferably from 5 to 15 xcexcm, such as from 8 to 10 xcexcm. For other displays, larger pixel sixes e.g. 300 xcexcm are more suitable.
In one preferred aspect, the pixels are arranged for polarized light emission. Suitably, the pixels are of the same color but have their polarization direction in different orientations. To the naked eye this would look one color, but when viewed through a polarizer some pixels would be bright and others less bright thereby giving an impression of 3D viewing when viewed with glasses having a different polarization for each eye.
The layers may also be doped with photoactive dyes. In aspects, the dye comprises a dichroic or pleachroic dye. Examples include anthraquinone dyes or tetralines, including those described in S. M. Kelly, Flat Panel Displays: Advanced Organic Materials, RSC Materials Monograph, ed. J. A. Connor, [2000]. Different dopant types can be used to obtain different pixel colors.
Pixel color can also be influenced by the choice of chromophore with different chromophores having more suitability as red, green or blue pixels, for example using suitably modified anthraquinone dyes.
Multicolor emitters are envisaged herein comprising arrangements or sequences of different pixel colors.
One suitable multicolor emitter comprises stripes of red, green and blue pixels having the same polarization state. This may be used as a sequential color backlight for a display which allows the sequential flashing of red, green and blue lights. Such backlights can be used in transmissive and reflective FLC displays where the FLC acts as a shutter for the flashing colored lights.
Another suitable multicolor emitter comprises a full color pixelated display in which the component pixels thereof have the same or different alignment.
Suitable multicolor emitters may be formed by a sequential xe2x80x98coat, selective cure, wash offxe2x80x99 process in which a first color emitter is applied to the aligned layer by a suitable coating process (e.g. spin coating). The coated first color emitter is then selectively cured only where pixels of that color are required. The residue (of uncured first color emitter) is then washed off. A second color emitter is then applied to the aligned layer, cured only where pixels of that color are required and the residue washed off. If desired, a third color may be applied by repeating the process for the third color.
The above process may be used to form a pixelated display such as for use in a color emissive display. This process is simpler than traditional printing (e.g. ink jet) methods of forming such displays.
According to another aspect of the present invention there is provided a backlight for a display comprising a power input; and a light emitter as described hereinbefore.
The backlight may be arranged for use with a liquid crystal display. In aspects, the backlight may be monochrome or multicolor.
According to yet another aspect of the present invention there is provided a display comprising a screen; and a light emitter or backlight as described hereinbefore.
The screen may have any suitable shape or configuration including flat or curved and may comprise any suitable material such as glass or a plastic polymer.
The light source of the present invention has been found to be particularly suitable for use with screens comprising plastic polymers such as polyethylene or polyethylene terephthalate (PET).
The display is suitable for use in electronic apparatus including a drive means therefor. The display is suitable for use in consumer electronic goods such as mobile telephones, hand-held computers, watches and clocks and games machines.
According to yet another aspect of the claimed invention there is provided a security viewer (e.g. in kit form) comprising a light emitter as described herein in which the pixels are arranged for polarized emission; and view glasses having a different polarization for each eye.
According to yet another aspect of the claimed invention there is provided a 3D viewer (e.g. in kit form) comprising a light emitter as described herein in which the pixels are arranged for polarized emission wherein the alignment of polarisation axis of each pixel is different; and a viewer having polarization characteristics aligned with those of the pixels.
According to yet another aspect of the claimed invention there is provided a method of forming a light emitter for a display comprising forming a photoalignment layer; and aligning a light emitting polymer on said photoalignment layer.
According to yet another aspect of the claimed invention there is provided a method of forming a multicolor emitter comprising applying a first color light emitter to the photoalignment layer; selectively curing said first color light emitter only where that color is required; washing off any residue of uncured first color emitter; and repeating the process for a second and any subsequent light color emitters.
All references herein are incorporated by reference in their entirety.